The proposed investigation is a study of the development of thymic lymphocytes from bone marrow stem cells. Bone marrow preparations, separated by velocity sedimentation, iso-pycnic sedimentation and counter- current distribution will be used to repopulate the thymus of irradiated mice and the kinetics of the appearance of thymic antigens will be studied both in vivo and cell culture. The development of mature, functional thymic lymphocytes will be correlated with these antigenic changes and the influence of a thymic humoral factor on these changes will also be evaluated. This humoral factor, a purified polypeptide of bovine origin has been termed thymopoietin. It has been shown to induce the early stages of thymic differentiation from splenic and bone marrow precursors and its effect on the development of immunologic competence will be studied.